Radiation diffractive materials based on crystalline colloidal arrays have been used for a variety of purposes. A crystalline colloidal array (CCA) is a three-dimensional ordered array of mono-dispersed colloidal particles. The particles are typically composed of a polymer, such as polystyrene. These colloidal dispersions of particles can self-assemble into ordered arrays (crystalline structures) having lattice spacings that are comparable to the wavelength of ultraviolet, visible or infrared radiation. The crystalline structures have been used for filtering narrow bands of selective wavelengths from a broad spectrum of incident radiation, while permitting the transmission of adjacent wavelengths of radiation. Alternatively, CCAs are fabricated to diffract radiation for use as colorants, markers, optical switches, optical limiters and sensors. Many of these devices have been created by dispersing particles in a liquid medium, whereby the particles self-assemble into an ordered array. The positions of the particles in the array may be fixed by mutual polymerization of the particles or by introducing a solvent that swells and locks the particles together.
Other CCAs are produced from a dispersion of similarly charged mono-dispersed particles in a carrier. The dispersion is applied to a substrate, and the carrier is evaporated to yield an ordered periodic array of particles. The array is fixed in place by coating the array with a curable polymer, such as an acrylic polymer, polyurethane, alkyd polymer, polyester, siloxane-contained polymer, polysulfide or epoxy-containing polymer. Methods for producing such CCAs are disclosed in U.S. Pat. No. 6,894,086, incorporated herein by reference. Alternatively, the particles may have a core-shell structure where the core is produced from materials such as those described above for unitary particles and the shell is produced from the same polymers as the core material with the polymer of the particle shell different from the core material for a particular array of the core-shell particles. Such core-shell particles and methods of their production are disclosed for example in U.S. Patent Application Publication No. 2007/0100026, incorporated herein by reference.
In these arrays of unitary particles or core-shell particles, the structures diffract radiation according to Bragg's law, wherein the radiation meeting the Bragg conditions is reflected while adjacent spectral regions that do not meet the Bragg conditions are transmitted through the device. While the wavelength of reflected radiation is in part determined by the effective refractive index of the array, the intensity of the reflective radiation is also proportional to the difference in refractive index between the particles in the array and the surrounding matrix.